(1) Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a technique which is effectively applied to a semiconductor device provided with a heat radiating structure.
(2) Description of Related Art
JP-A-2006-261336 (patent document 1) discloses a technique forming a heat radiating structure for heat radiating an electronic device by using a thermal conduction sheet formed as a sheet shape by an elastic material in which a surface has an adhesive property, covering one surface of the thermal conduction sheet by a perforated sheet, sticking the other surface to the electronic device, pressing a heat radiating thread to the perforated sheet, and pushing out the surface of the elastic material of the heat conduction sheet from a hole of the perforated sheet on the basis of a pressing so as to stick the heat radiating thread and the thermal conduction sheet.
JP-A-2004-259977 (patent document 2) discloses a technique that a thermal conduction member is deformed so as to relax a stress between a casing and a semiconductor chip by accommodating a substrate mounting the semiconductor chip thereon in the sealed casing, forming a structure pinching the thermal conduction member to a gap between the semiconductor chip and an inner wall of the casing, and providing a structure having concavity and convexity at least in one of a portion opposing to the semiconductor chip in the inner wall of the casing and the semiconductor chip.
JP-A-2001-68607 (patent document 3) discloses a technique suppressing a deflection of a circuit board generated by a load applied to the circuit board by compressing a soft high thermal conduction body at a time of interposing the high thermal conduction body between the circuit board and a heat radiating plate so as to heat radiate.
FIGS. 8 and 9 are cross sectional views showing a heat radiating structure in an optical transmitter and receiver (an optical transmission module) considered by the inventors of the present invention, and respectively show different cross sections. The heat radiating structure shown in FIGS. 8 and 9 is constituted by a semiconductor package 102 mounted on a printed circuit board 101, a thermal conduction sheet 103 arranged on an upper surface of the semiconductor package 102, and a metal case 105 provided with a heat radiating fin 104 radiating a heat transmitted from the thermal conduction sheet 103 to an atmospheric air.
The thermal conduction sheet 103 employs a sheet having an adhesive property on a surface which comes into contact with the semiconductor package 102 and the metal case 105, and having an elasticity in such a manner that a thickness is changed in correspondence to a gap between the semiconductor package 102 and the metal case 105.
In the example shown in FIGS. 8 and 9, there is shown the semiconductor package 102 having a ball grid array structure using a solder ball in a connection portion to the printed circuit board 101.
In accordance with one example of an assembling procedure of the heat radiating structure shown in FIGS. 8 and 9, the heat radiating structure is assembled by screw fixing the printed circuit board 101 to which the semiconductor package 102 is connected by a solder to a metal case 106 forming a pair together with the metal case 105 by using a screw 107 and a screw 108, arranging the thermal conduction sheet 103 on the upper surface of the semiconductor package 102, and thereafter putting the metal case 105 provided with the heat radiating fin 104 thereon so as to fix to the metal case 106 by a screw 109 and a screw 110. The screw 109 and the screw 110 are fitted to a through hole 111 provided in the metal case 105 and a thread hole 112 provided in the metal case 106, whereby the metal case 105 and the metal case 106 are fixed.
A gap between the metal case 105 and the semiconductor package 102 becomes narrow at a time of fixing the metal case 105 provided with the heat radiating fin 104 to the metal case 106 by the screw 109 and the screw 110. The thermal conduction sheet 103 is adjusted to a thickness corresponding to the gap between the metal case 105 and the semiconductor package 102 by utilizing an elasticity of the thermal conduction sheet 103 itself.
There is a case that a thickness of the thermal conduction sheet 103 can not be rapidly changed in the case that an environmental (atmospheric) temperature is low or in some pressurizing speed caused by a screwing even if the elasticity is provided. In the case that the heat radiating structure shown in FIGS. 8 and 9 is assembled under the condition mentioned above, the pressure applied from the metal case is not relaxed by the thermal conduction sheet 103, and a mechanical stress is applied to the semiconductor package 102. If an excessive mechanical stress is applied to the semiconductor package 102, there is a risk of breaking the semiconductor package 102 and the connection portion between the semiconductor package 102 and the printed circuit board 101. Further, since the mechanical stress is transmitted to the printed circuit board 101, a strain is generated in the printed circuit board 101, and there is a risk of damaging the surface mounted parts mounted around the semiconductor package 102 such as a resistance, a condenser and the like.
The thermal conduction sheet 103 goes on keeping an elastic stress generated at a time of assembling after a time elapse except just after assembling the heat radiating structure shown in FIGS. 8 and 9. On the basis of the elastic stress, the mechanical stress is continuously applied to the semiconductor package 102 and the printed circuit board 101 for a long time period, and there is a risk of deteriorating a characteristic and lowering a service life of the surface mounted parts mounted on the printed circuit board 101 including the semiconductor package 102.
A graph shown by a solid line in FIG. 10 corresponds to one example of a characteristic view showing a change with time of the mechanical stress applied to the semiconductor package 102 in the case of pressurizing the thermal conduction sheet 103 by the metal case 105 in the heat radiating structure shown in FIGS. 8 and 9. A vertical axis of the graph in FIG. 10 indicates the mechanical stress applied to the semiconductor package 102, and a horizontal axis indicates a time on the basis of a logarithmic display.
In the case of rapidly pressurizing the thermal conduction sheet 103 by the metal case 105, since the thermal conduction sheet 103 can not follow the rapid change, the mechanical stress is applied to the semiconductor package 102 without being relaxed. A rapid increase of the mechanical stress is expressed in the graph in FIG. 10. Since the thermal conduction sheet 103 thereafter familiarizes itself with the pressure, the mechanical stress is slowly reduced in accordance with a time elapse, however, the mechanical stress is continuously applied to the semiconductor package 102 on the basis of a residual stress of the thermal conduction sheet 103, although it is smaller than the mechanical stress at a time of pressurizing.
FIG. 11 shows an example of a heat radiating structure considered by the inventors of the present invention as a technique suppressing the mechanical stress generated by the heat radiating structure shown in FIGS. 8 and 9. The heat radiating structure shown in FIG. 11 employs a heat conduction sheet 113 in which a concavo-convex structure is provided in the surface. The thermal conduction sheet 113 is structured such that the concavo-convex structure is provided on a surface of the thermal conduction sheet 103 by embossing the concavo-convex structure on the thermal conduction sheet 103 having the flat surface used in the heat radiating structure shown in FIGS. 8 and 9 by using a mold having concavity and convexity, as shown in FIG. 12.
In the heat radiating structure using the thermal conduction sheet 113 shown in FIG. 11, even in the case that the metal case 105 is fixed by the screws 109 and 110, the excessive mechanical stress is hard to be applied to the electronic part mounted on the printed circuit board 101 including the semiconductor package 102 on the basis of the collapse of the concavo-convex structure of the surface provided in the thermal conduction sheet 113.
However, since the thermal conduction sheet 113 shown in FIG. 11 is provided with the concavo-convex structure on the surface, there is a hard handled point such as a strength of the concave portion is weak and the concave portion is scattered at a time of gripping the thermal conduction sheet. The concavo-convex structure tends to collapse at a time of gripping the thermal conduction sheet by a finger, tweezers or the like, and there is a case that the mechanical stress at a time of assembling the optical transmission module can not be relaxed in this state.
In the case of the technique disclosed in JP-A-2004-259977 (patent document 2), since the structure is made such that the concavity and convexity are formed at least one of the portion opposing to the semiconductor chip in the inner wall of the casing and the semiconductor chip, there is a case that an air is sealed between the concavity and convexity and the thermal conduction member if the thermal conduction member is warped. In the case that the air is sealed as mentioned above, there is a risk that an adhesiveness with the thermal conduction member is lowered, or a heat radiating performance is lowered.
In the case of the technique disclosed in JP-A-2001-68607 (patent document 3), since the structure is made such that the through hole is provided in the heat radiating plate, there is a risk that a work for forming the through hole becomes hard. Even in the case that the through hole is provided, there is a risk that the heat radiating fin can not be provided in the heat radiating plate due to the structure of the heat radiating plate. Further, there is a risk that an electromagnetic wave leaks out to an external portion from the through hole. Further, in the case that the optical transmitter and receiver is arranged in the device such as a rooter or the like, there is a risk that the other parts within the device are erroneously actuated by the leaking electromagnetic wave. Further, there is a risk that a foreign material enters into the optical transmitter and receiver from the through hole.